He was adopted by Hideyoshi and called himself Hashu Hidetoshi (羽柴 秀俊) and Shusen (秀栓). He was then again adopted by Kobayakawa Takakage and renamed himself Hideaki. Because he had gained the rank of Saemon no Kami (左衛門督) or in China Shikkingo (執金吾) at genpuku and held the title of Chunagon(中納言), Hideaki was also called Kingo Chunagon (金吾中納言).

During the Battle of Keicho he led reinforcements to rescue Ulsan Castle from the Ming army. Fighting on the front line with a spear, he managed to capture an enemy commander and successfully broke the siege. However, Hideyoshi saw the danger of a reckless charge by the general commanding an army and deprived him of his domain, Chikugo after returning. Kobayakawa, angered by this, believed the lie circulated by Tokugawa Ieyasu that this had been the doing of a jealous Ishida Mitsunari. He never forgot or forgave Mitsunari and worked to undermine his position. Moreover, Kobayakawa was known to attack women and children during the campaign, an act that is despised by many of his fellow commanders.

Before the battle of Sekigahara, Kobayakawa happened to be in Osaka and acted as though he would go along with Mitsunari, even though he had intended to betray him, having secretly communicated with Ieyasu. Knowing Kobayakawa held ill feelings, Mitsunari and Ōtani Yoshitsugu promised him two additional domains around Osaka and the position of kampaku (until Toyotomi Hideyori grew old enough to rule) if he helped them to victory.

Even after the battle began, Kobayakawa kept his intentions hidden. Ieyasu's force (east) was not faring well against Mitsunari from the west; Ukita Hideie was winning against Fukushima Masanori and Ōtani Yoshitsugu was also winning against Tōdō Takatora. Kobayakawa was hesitant to participate with either side. Ieyasu ordered troops to fire blanks against the Kobayakawa troops to force them into action. Kobayakawa then ordered an attack on the Otani troop and while this attack was beaten back temporarily, his action forced the other armies who had pledged betrayal to also turn. The battle was over within a day.

Kobayakawa also had success in the mopping up operations that followed, defeating Mitsunari's father, Ishida Masatsugu in the siege of Sawayama. Once the dust had settled, Kobayakawa was given the defeated Ukita clan's former fiefdoms of Bizen and Mimasaka, for a total of 550,000 koku. However, Kobayakawa suddenly died two years later after supposedly going mad, and with no one to succeed him, the Kobayakawa clan disbanded, and his fiefdoms were absorbed by the neighboring Ikeda clan.

In the video game Kessen he is portrayed as a pathetic general dressed in splendid ochre armor, while in the mangaKagemusha Tokugawa Ieyasu he is seen with a childish mind. In the animeSanada Juyushi Special he is portrayed sympathetically due to the many disappointments in his childhood, and also because of the broken promise that prevented him from becoming Hideyoshi's heir when Hideyori was born.

In Samurai Warriors 2 (Sengoku Musou 2), he is portrayed as young, inexperienced, and very indecisive. Shima Sakon refers to him as "Bitty Baby Kobayakawa". As mentioned previously in this article, he panics and joins the Tokugawa ranks when he is fired upon by Magobei Fuse, an officer of the Eastern Army. He is also present in Nene's Dream Stage, acting as her 'son', when in fact he feared Nene more than the rifles that supposedly drove him to defect to the Eastern Army.

In the game Sengoku Basara 3, he is portrayed as a coward and a glutton, and refers to himself as "The Gourmet General". During his boss battle, the player will face him in a fortress that surrounds a gigantic nabe pot. He also wears armor that makes him look like a rhinoceros beetle, his helmet represents the horn, and the nabe pot he wears on his back represents the shell. Also in one of the Sekigahara scenario's Tadakatsu Honda fires at Hideaki with his cannons to make him go over Ieyasu's side, which is a reference to the decision made by Ieyasu at the actual battle.